An American Passive Home: Page 3 of 4

Intermediate

Inside this Article

Pura Vida demonstration home.
Pura Vida, a common saying in Costa Rica meaning ”pure life,” is the name of this demonstration home built by the author and his wife in Oregon, Illinois, in 2007.
Pura Vida interior.
A timber-frame structure, Pura Vida demonstrates an efficient home that looks and feels conventional.
Insulated concrete forms.
Insulated concrete forms include the support that keeps the forms from being pushed apart during the concrete pour.
Large south-facing windows.
Large south-facing windows, specified for their high solar heat gain coefficient, admit ample sunlight to warm the house on sunny winter days.
The “earth room” under construction.
The “earth room” under construction. Once complete, this space will provide a source of free heating and cooling.
Enerboss
A 4.5 kW Marathon water heater provides hot water to the heat-exchanger coils within this all-in-one Enerboss, a heating, filtration, and heat-recovery ventilation system.
A 4.5 kW Marathon water heater
This 4.5 kW Marathon water heater provides hot water to the heat-exchanger coils within the all-in-one Enerboss, a heating, filtration, and heat-recovery ventilation system.
An air-to-water heat pump.
An air-to-water heat pump provides domestic hot water.
Wind-electric generator.
A small wind-electric generator offsets a small portion of the home’s electricity use. It would produce much more on a tower tall enough to clear all obstructions in the area.
Two pole-mounted PV arrays.
Two pole-mounted PV arrays offset about 27% of the home’s electricity use.
Battery bank.
A battery bank provides backup power in the event of a utility outage.
OutBack MATE3
An OutBack MATE3 keeps tabs on the RE system.
OutBack Radian series inverter.
This OutBack Radian series inverter and load center also serves as an AC and DC enclosure. The system’s charge controller sits to the right.
Wood floors and exposed posts and beams.
Wood floors and exposed posts and beams lend warmth and beauty to this efficient home.
Pura Vida demonstration home.
Pura Vida interior.
Insulated concrete forms.
Large south-facing windows.
The “earth room” under construction.
Enerboss
A 4.5 kW Marathon water heater
An air-to-water heat pump.
Wind-electric generator.
Two pole-mounted PV arrays.
Battery bank.
OutBack MATE3
OutBack Radian series inverter.
Wood floors and exposed posts and beams.

Earth Room

In addition to capturing solar gain, Pura Vida takes advantage of another source of free heat (and cooling): from the relatively constant temperature of the ground. In Europe, earth tubes are often buried around the foundation of homes to provide tempered fresh air, but in the United States, there has been concern that earth tubes can grow mold or mildew. With Pura Vida, and subsequent buildings I have designed, an “earth room”—a modified approach to earth tubes—has been successful.

In Pura Vida, the earth room lies below the front porch. A short, 12-inch-diameter tube brings fresh outdoor air into one end of the earth room. The air flows the length of the 48-foot-long room where it is preheated (in the fall and winter) or precooled (spring and summer) through direct contact with the concrete walls prior to entering a heat recovery ventilator (HRV). The earth room is like a large thermal battery, storing heat in the summer for use in the fall and winter and storing “coolness” in the winter for use in the spring and summer. The earth room eliminates the need to use a conventional mechanical heating system for about two months of the year (October and November) and eliminates the need to run a cooling system from mid-May to mid-June. Throughout the rest of year, the earth room significantly reduces the heating and cooling loads.

After passing through the earth room, the air enters the Nu-Air Ventilation Enerboss, a complete heating, filtration, and HRV system. An efficient fan constantly pulls air from the bathrooms and kitchen, which is exhausted, while the same amount of fresh air is evenly distributed throughout the home by an airflow-balanced high-velocity duct system. The system has operated flawlessly for the past five years. The Enerboss system requires an external source of hot water for heating the air to be distributed. In Pura Vida, we use a 4.5 kW Marathon water heater to provide hot water to the heat exchanger coils within the Enerboss. Air conditioning is provided by a 3-ton, 16 SEER Lennox Elite. The evaporator for the AC unit is mounted on top of the Enerboss.

Visitors to Pura Vida often comment how fresh the air is and how quiet it is within the home. The comfort provided by constantly filtered fresh air moving throughout the home makes it difficult to go back to living with a conventional HVAC system.

Domestic Hot Water

A small Nyle Systems air-to-water heat pump mounted on a wall in the earth room provides domestic water heating. A timer is programmed to turn it on in the evenings when the time-of-use electricity price is $0.02 to $0.03 per kWh. The hot water is stored in a well-insulated 105-gallon Marathon water heater for use during the day. The average monthly cost for domestic hot water is about $5. (The timer is bypassed when we have guests or need to use more hot water.)

We also have a GFX Technologies wastewater-to-water heat exchanger. Although the concept of recovering the energy in hot water going down the drain is an interesting one, the high cost of copper makes this technology too expensive to be cost-effective for the amount of preheated water the unit provides.

Demonstrated Performance

Six Lascar temperature and humidity sensors were placed throughout the home, outside, and in the earth room. Data collected from these sensors every 30 minutes for the past five years—along with energy use data collected from several TED (The Energy Detective) units and our utility bills—have validated the home’s performance.

The all-electric home’s advantage is that we can measure and directly compare the energy use for every appliance and system. Having collected data on the efficiency of the air-to-water heat pump in the earth room for providing domestic hot water, I plan to modify the heating system to include a second heat pump, instead of the existing water heater, for space heating. This should reduce the electricity demand for space heating by at least 60%.

When the home was built, we also decided to sign up for time-of-use utility metering. This utility billing method provides us with cheaper energy during off-peak times, and more expensive energy during peak times. The risk we took with this decision was that if we needed to use large amounts of energy during peak times—like running air conditioning midday during the summer—the cost could be significantly higher. The table (upper right) shows our average annual energy use for the various portions of the property along with an approximate annual cost for each.

Because of the relatively high upfront cost of renewable electricity systems, we felt it was important to first design and build a home that is as efficient as possible—and then get a good understanding of the home’s energy demands. A year after the home was completed, we applied for and received a state grant to measure the effectiveness of residential small wind systems.

We installed a 2.4 kW Southwest Windpower Skystream 3.7 turbine on a 60-foot tower (at the time, we did not understand that height is too short for nearly all applications). The utility account was converted to net metering so that we could sell our excess energy back to Commonwealth Edison. The energy production from the Skystream was monitored through a Zigbee data system and logged on a computer.

Comments (1)

zap101's picture

Yours is a fine home. Sharing the bad and the good outcomes is useful. The break out of costs of the insulation is helpful adding a cost per sqft/ payback might show the bang for your buck sort of speak. For example the under slab insulation cost per sqft/ pay back # of years in energy saving verses non insulated slab.

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